### What this PR does / why we need it?
Previously, the mask construction process created multiple tensors of
size (max_model_len, max_model_len). When max_model_len reached 128k,
single GPU host memory usage exceeded hundreds of GB, causing process
OOM crashes. This update optimizes the mask generation to significantly
reduce memory consumption.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
CI pass.
- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0
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Signed-off-by: Jade Zheng <zheng.shoujian@outlook.com>
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